Mass spectrometer detector and system and method using the same

ABSTRACT

An ion detector for secondary ion mass spectrometer, the detector having an electron emission plate coupled to a first electrical potential and configured to emit electrons upon incidence on ions; a scintillator coupled to a second electrical potential, different from the first electrical potential, the scintillator having a front side facing the electron emission plate and a backside, the scintillator configured to emit photons from the backside upon incidence of electrons on the front side; a lightguide coupled to the backside of the scintillator and confining flow of photons emitted from the backside of the scintillator; and a solid-state photomultiplier coupled to the light guide and having an output configured to output electrical signal corresponding to incidence of photons from the lightguide. A SIMS system includes a plurality of such detectors movable arranged over the focal plane of a mass analyzer.

BACKGROUND 1. Field

This disclosure relates to the field of secondary ion mass spectrometersand, more specifically, for an improved mass spectrometer detector and asystem and method using the detector for extending the dynamic range ofparticle detection in such an apparatus and improving detectionaccuracy.

2. Related Art

Secondary ion mass spectrometry (SIMS) is a technique used to analyzethe composition of a sample by sputtering the surface of the sample witha focused primary ion beam and collecting and analyzing secondary ionsejected from the sample. The mass to charge ratios of these secondaryions are indirectly measured by separating them spatially with a massanalyzer, so as to determine the elemental, isotopic, or molecularcomposition of the sample. The secondary ions are detected generallyusing one of two methods: a Faraday cup or an electron multiplier. AFaraday cup measures the ion current hitting a metal cup, and issometimes used for high current secondary ion signals. In an electronmultiplier an impact of a single ion starts off an electron cascade,resulting in a pulse of electrons which is recorded directly. Theelectron multiplier can comprise a series of individual dynodes, achannel electron multiplier, or a microchannel plate.

Generally, mass spectrometers operate in two regimes: a counting modeand an analog mode. As illustrated in FIG. 1A, digital mode is effectiveup to about 1×10⁶ ions/second, depending on the characteristics of thedetector. Analog mode is effective above about 1×10⁸ ions/second. Asshown in FIG. 1A, a first problem is that a detection gap exists betweenabout 1×10⁶ ions/second to about 1×10⁸ ions/second, since the current istoo high for the digital mode to separate the individual events, but isinsufficient to provide a reading for estimating the arriving ions.Another problem, that is not manifested in FIG. 1A, is that thecorrelation between the digital mode and analog mode is not accurateand/or repetitive. Thus, when switching between digital and analogmodes, one gets an artificial “jump” in the reading. These problems canbe more fully understood from the following.

In pulse counting mode, each individual particle is assumed to producean “event” in the detector output signal, which can be identified andcounted. Pulse counting mode has the advantage of being inherentlyquantified since each incoming particle is assumed to produce a singledetected event. (It is important to note that, in this case, we are notattempting to characterize each event for quantitative properties suchas pulse height, total charge, etc.) The two limitations of pulsecounting mode are that to be properly identified (1) each incomingparticle (or at least a sufficient percentage of them) needs to producean event over the detection threshold and that (2) any two particlesneed to arrive with a separation such that, given the pulse shape andlength, they can be independently detected. Therefore, the pulsecounting mode is restricted to the detection of relatively low rates ofion arrival. The ion arrival rate is assumed to be described by aPoisson process and the generation of electrons on the conversion dynodeand photons in the scintillator are also described by statisticalprocesses.

Although the average ion arrival rate may be such that ions pulses aresufficiently separated in time as to be independently detectable, thedistribution of ion arrival times dictates that some percentage willoccur too close together in time for this to be the case. For thisreason, the maximum ion arrival rate at which counting can be used mustinclude a specification for what percentage of events can be missed dueto pulse stack-up. This percentage should be such that valid statisticalestimation of the true count can be made despite the non-detection ofsome events.

Regarding analog mode, SIMS instruments generally use a Faraday cupapproach in which the current is proportional to the arrival rate ofions. However, at rates below about 1×10⁸ ions/second, the current istoo low to measure. Using the electron multiplier in an analog mode isnot accurate. This is due to the statistics of secondary electronemission in the dynodes, channel walls, and/or photon emission in thescintillator. In addition to these short term variations, there willalso be long term variations due to changes in the surface condition ofthe conversion dynode, aging of the scintillator, PMT photocathode anddynodes, etc. For these and other reasons, when operating in analog modeit is not possible to use the integrated area under all the pulses in anacquisition to measure incoming ion flux unless the percent averagepulse area per incoming ion is accurately known. For this reason,accurate measurement of ion arrival rates above a certain value,determined by the properties of the dynode, scintillator, and detector,is not possible using an electron multiplier.

What is needed, therefore, is an improved detector having fasterdetection speed to enable separation of incoming ions. Also needed is amethod to calibrate the average pulse area per incoming ion, thusenabling an accurate analog-mode measurement for ion arrival rate.

SUMMARY

The following summary of the invention is included in order to provide abasic understanding of some aspects and features of the invention. Thissummary is not an extensive overview of the invention and as such it isnot intended to particularly identify key or critical elements of theinvention or to delineate the scope of the invention. Its sole purposeis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented below.

Aspects of the invention provide an improved ion detector for use inmass spectrometry. The improved detector has a fast rise and decay time,thus enabling better separation of detection events in digital mode. Thedetector also provides improved signal to noise ratio, thus enhancingdetection performance.

According to aspects of the invention, an ion detector for SIMS isprovided, comprising: an electron emission plate coupled to a firstelectrical potential and configured to emit electrons upon incidence onions; a scintillator coupled to a second electrical potential, differentfrom the first electrical potential, the scintillator having a frontside facing the electron emission plate and a backside, the scintillatorconfigured to emit photons from the backside upon incidence of electronson the front side; a lightguide coupled to the backside of thescintillator and confining flow of photons emitted from the backside ofthe scintillator; and a solid-state photomultiplier coupled to the lightguide and having an output configured to output electrical signalcorresponding to incidence of photons from the lightguide. Thesolid-state photomultiplier may be a silicon photomultiplier or an arrayof avalanche photodiodes having outputs coupled together to provide asingle electrical signal. The lightguide may have a reflective surfacewhich directs the photons onto the solid-state photomultiplier. Thesolid-state photomultiplier may be coupled to a rear surface of thelightguide.

In various embodiments, several detectors are provided in spatialseparation, such that several species can be detected simultaneously. Indisclosed embodiments, the detectors are movable, such that by spatialpositioning of a detector, the same detector can be used for detectingdifferent species. Therefore, when the material composition is knownbeforehand, e.g., various material layers on a semiconductor wafer, thedetectors can be arranged spatially to detect the species expected fromthat sample. Then the counting from each detector can be used to verifythat the sample indeed has the expected species at the expectedconcentration and depth, e.g., at the expected doping levels. Similarly,the counting from the different detectors can be used to study thecomposition of the interface between two layers of different materials.

Thus, according to disclosed aspects, a secondary ion mass spectrometersystem is provided, comprising: an ion source providing a primary ionbeam; a secondary ion extractor collecting secondary ions sputtered froma sample by the primary ion beam; beam forming optics receiving thesecondary ions from the secondary ion extractor and forming a secondaryion beam; a mass analyzer forming a trajectory onto a focal plane; aplurality of ion detectors movably positioned along the focal plane;wherein each of the ion detectors comprises: a first section configuredto emit electron upon impingement of secondary ions; a second sectionconfigured to emit photon upon impingement of electrons; a third sectionconfigured to emit an electrical signal upon impingement of photons;and, a lightguide coupled on one end to the second section and onanother end to the third section.

In various embodiments the secondary ion mass spectrometer may furthercomprise a spectrometer configured to allow only secondary ions within adesired band of mass-to-charge ratio to pass through. The mass analyzermay comprise a spectrometer; a plurality of quadruples; at least onehexapole; and, a main magnet. The first section may comprise an electronemission plate coupled to a first electrical potential and configured toemit electrons upon incidence on ions. The second section may comprise ascintillator coupled to a second electrical potential, different fromthe first electrical potential, the scintillator having a front sidefacing the electron emission plate and a backside, the scintillatorconfigured to emit photons from the backside upon incidence of electronson the front side. The third section may comprise a solid-statephotomultiplier coupled to the light guide and having an outputconfigured to output electrical signal corresponding to incidence ofphotons from the lightguide.

Disclosed aspects also provide a system wherein counting mode and analogmode can be operating simultaneously. According to disclosed embodiment,two signals are obtained from the detector, both of which correlate tothe same detection event. The two signals are applied to two channels;one channel includes electronic devices optimized for the counting modedetection, while the other channel includes electronic devices optimizedfor analog mode detection. Both channels can be operated simultaneouslysince the two signals, while correlated to the same detection events,are generated separately.

Disclosed embodiments provide a method for simultaneously operatingcounting mode and analog mode at a photomultiplier of a SIMS apparatus.The method comprises coupling a first signal line to thephotomultiplier; configuring a digital event counter to receive an inputfrom the first signal line and provide a discrete event count outputcorresponding to the input; coupling a second signal line to thephotomultiplier independently of the first signal line; configuring ananalog integrator to receive input from the second signal line andprovide integrated analog output simultaneously with the discrete eventcount output from the digital event counter. Coupling the first signalline to the photomultiplier may include coupling the first signal lineto a bias input line of the photomultiplier, while coupling the secondsignal line may include coupling the second signal line to an outputline of the photomultiplier. Coupling the first signal line to thephotomultiplier may include coupling the first signal line to a dynodeof the photomultiplier, while coupling the second signal line mayinclude coupling the second signal line to a collector of thephotomultiplier.

According to further aspects, a calibration method is provided utilizingsimultaneous signals of the analog and digital modes. Specifically, anion arrival rate is produced by some means to be within a desiredregime. Then pulse counting mode is used to produce a number of pulseswhile simultaneously the analog signal is integrated to produce a totalarea over all pulses. The signals from both channels are used to forcalibration. For example, in one embodiment the signals are used togenerate a scaling factor to be applied when switching between countingand analog modes. In another example, the integrated pulse area isdivided by the number of pulses to thereby render the average area perpulse, which is used as a calibration factor.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify the embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate major features of the exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

FIG. 1A is a schematic illustration digital and analog detection regimesaccording to the prior art.

FIG. 1B is a schematic illustration of a system according to anembodiment of the invention.

FIG. 2 is a schematic illustration of an ion detector according to anembodiment of the invention, while FIG. 2A is a schematic illustrationof an ion detector according to another embodiment of the invention andillustrated other features that may implemented in any embodimentdisclosed herein.

FIG. 2B is a schematic illustration of circuits using isolator,according to disclosed embodiments.

FIGS. 2C and 2D are schematic illustration of circuits using isolator,according to disclosed embodiments.

FIG. 2E is a schematic of a circuit using replicator, according todisclosed embodiment.

FIG. 3 is a schematic illustration of an ion detecting scheme accordingto one embodiment of the invention.

FIG. 4 illustrates a condition wherein the ion sputtering causes acrater with different sputtering rate at the edges and at the center ofthe crater.

DETAILED DESCRIPTION

Various embodiments will now be described with reference to thedrawings. Each embodiment may be described with reference to one or morefeatures or elements, but it should be appreciated that each describedfeature and/or element may be used in conjunction with other featuresand/or elements of other embodiments. That is, the features and elementsmay be interchangeable and/or additive among the various embodiments.

FIG. 1B schematically illustrates the construction of one embodiment. Asample 100 is to be examined to determine the material composition ofthe sample. An ion source 110 is used to generate a focused ion beam115, which is made to impinge on sample 100, so as to eject secondaryions from the sample. The extraction and beam shaping optical elements117 collect charged secondary ions and forms secondary ion beam 120 thatis delivered into detection section 130. Detection section 130 includesa mass analyzer 135, which changes the trajectory of the secondary ions120. Since different ions have different mass-to-charge ratio, theeffect of the mass analyzer on their trajectory is different, such thatdifferent ions assume different spatial trajectory path (three of whichare shown). A plurality of detectors 140 are positioned at differentspatial locations inside the detection section 130, such that eachdetector is positioned to intercept ions of certain mass-to-chargeratio. Each detector 140 is provided with mechanized positioning means,such that the spatial position of each detector 140 can be changed, soas to detect ions of different mass.

FIG. 2 schematically illustrates an embodiment of a detector 240according to one embodiment. The detector 240 of FIG. 2 is ascintillator coupled with a solid-state photomultiplier, providingenhanced dynamic range and fast signal response time, in that it hasfast decay time to enable separation of rapidly arriving ions. As such,the detector converts ions stream into an electron stream, then convertsthe electron stream into a photon stream, and finally converts thephoton stream into an electrical signal.

Secondary ions enter the enclosure 241 via window 242 and hit electronemission plate 243. Electron emission plate is biased by potential V1,to thereby eject electrons upon being hit by ions. The electrons areemitted in various directions, but by applying a potential V2 toscintillator plate 244, the electrons are focused onto the scintillatorplate 244. When the electrons hit the front side of the scintillatorplate 244, the scintillator plate 244 emits photons from the backsurface. A tubular light guide 245 is optically coupled to thescintillator plate 244 and may have various cross-section shape asneeded, e.g., rectangular or square cross-section. The light guide 245may be made of light conducting transparent material. The photons travelwithin light guide 245 and remain confined inside the light guide bytotal internal reflection. In the embodiment of FIG. 2, light guide 245has a reflective surface 246 (forming a prism), which directs thephotons (e.g., by total internal reflection) into solid-statephotomultiplier 247. The solid-state photomultiplier 247 generateselectrical signal corresponding to the photons hitting the solid-statephotomultiplier. In this embodiment, the solid-state photomultiplier maybe a silicon photomultiplier (also referred to as SiPM) and may comprisean array of avalanche photodiodes (APD). The signal from the photodiodescan be summed up to provide a single output signal having sharp pulses251 upon detection of arriving photons, as schematically shown in FIG.2. The output of the solid-state photomultiplier 247 can be used tocount the pulses, corresponding to arriving ions of a specific species,depending on the spatial position of detector 240.

FIG. 2A schematically illustrates an embodiment of a detector 240according to another embodiment. The elements of the embodiment of FIG.2A are similar to that of FIG. 2, except that the light guide 245 doesnot have a reflecting surface forming a prism. Rather, the solid-statephotomultiplier 247 is attached directly to the back of the light guide245. Other than that, the detectors of FIG. 2 and FIG. 2A are the same.

FIG. 2A also illustrates various features relating to the collection ofdetection signals. Any one of these features can be implemented in thesame way in the embodiment of FIG. 2, or any other embodiment disclosedherein.

A first feature illustrated in FIG. 2A is the simultaneous operation ofcounting and analog modes. This will be first explained by reference tothe elements shown inside the dashed callout. In the prior art, thesystem is operated either in counting mode (when the ions arrival rateis sufficiently low to enable discrimination of individual arrivalevents) or in analog mode (when the arrival rate is too high to enableseparation). Another reason that both modes cannot operatesimultaneously in the prior art is that the signal is too weak, so itcannot be split into two channels. In essence, since the output signalis made of electron current, splitting that signal into two channelswould mean that each channel would have fewer electrons (i.e., lowercurrent) than what was originally output by the detector. Consequently,when the arrival rate is low, the detection signal has insufficientelectrons to be able to share among the two channels.

However, embodiments disclosed herein enable a system wherein both modescan operate simultaneously. Referring back to FIG. 2A, the dashedcallout illustrates one embodiment wherein two signals are obtained fromthe detector and are fed into two channels. The top channel is thecounting mode channel, while the lower channel is the analog channel.The two output signals are correlated to the same detection events, suchthat both channels simultaneously detect the same amplified ion flux. Inthe top channel, the signal is conditioned by capacitor 258, amplifiedby amplifier 260, and then the amplified signal is input todiscriminator 262, which outputs a digital signal corresponding to theinput from amplifier 260. The digital signal is input to counter 264 tocount the detection events. A reset signal enables resetting of thecounting. In the bottom/analog channel, the signal from the detector isintegrated by integrator 261 and is then amplified by amplifier 263. Theintegrator may be reset by a reset signal. These two channels can beoperated simultaneously or one at a time.

As noted above, the output signal of the detector is generallyinsufficient to operate both counting mode and analog modesimultaneously. The following description details various embodimentsthat overcome this problem. It is noted that the solutions detailedherein can be implemented in conjunction with the inventive detectordisclosed herein, or with conventional detectors.

A first example is illustrated in the dash-dot callout of FIG. 2A. Inthe dash-dot callout APD 247′ schematically represents the solid-statephotomultiplier 247. The APD 247′ is biased by voltage potential 270,through an isolator 272. A first output is connected to the bias lineleading from the voltage potential 270 to the APD 247′. This firstoutput line is led to the counting mode channel. On the other hand, asecond output signal is obtained from the output of the APD 247′. Thesecond output line is led to the analog mode channel. When a detectionevent occurs, current will be flowing out of the output of the APD 247′which will be directed to the analog channel for sensing.Simultaneously, there will be a voltage drop on the bias line, whichwill be detected by the counting mode channel. By this configuration,the two channels sense the same event simultaneously, even though thetwo channels need not share the same output signal.

Another arrangement that is similar to that of FIG. 2A is illustrated inFIG. 2B. APD 247′ schematically represents the solid-statephotomultiplier 247. The APD 247′ is biased by voltage potential 270 andis connected to replicator 273. The signal from replicator 273 is outputto the channel with capacitor 258 and amplifier 260, as in theembodiment of the dash-dot callout of FIG. 2A. Another line is connectedto have the signal integrated by high gain integrator 261′, while athird line is connected to have the signal integrated by low gainintegrator 261′.

An alternative configuration is illustrated in the two dot-dash callout.The arrangement is similar to that of the dot-dash callout, except thatthe line of the counting channel is not directly connected to the biasline from the voltage potential 270, but is rather coupled to it viatransformer 259. Other than that the two arrangements are the same.

Examples of circuits using isolator are illustrated in FIGS. 2C-2D. Inthese embodiments the pulse and analog signal paths originate onopposite ends of the solid-state photomultiplier 247, specifically, thepulse signal path is couple to the cathode side of the photodiode, whilethe analog signal path is coupled to the anode side of the photodiode.In both circuits, the operating voltage of the solid-statephotomultiplier 247 is regulated by a transistor 248 with respect to thevirtual ground of an op amp 254 (shown connected in the invertingconfiguration, so that the inverting input of the op-amp is coupled tothe anode of the photodiode). For the circuit illustrated in FIG. 2C,the op amp 254 provides the analog signal and the voltage drop in thetop resistor 257 (caused by photo-current) becomes the pulsed signal. Inthe circuit of FIG. 2D, instead of a voltage drop in a resistor, thecurrent pulse is sent through a pulse transformer 259.

In other words, the embodiment of FIG. 2C provides an isolator circuitfor ion detector having a photodiode, comprising: a bias transistorhaving its emitted coupled to a cathode of the photodiode and acollector coupled to a resistor; a pulse signal path coupled between thecollector and the resistor; an analog path coupled to an anode of thephotodiode, wherein the analog path comprises an operational amplifier(op-amp) having one input coupled to the anode of the photodiode, asecond input coupled to ground, and an output providing an analog signalresponse corresponding to the photodiode sensing photons. In theembodiment of FIG. 2C the op-amp is coupled to the anode of thephotodiode in an inverting configuration, wherein the inverting input ofthe op-amp is coupled to the anode of the photodiode. Also, a feedbackresistor is coupled between the inverting input and the output of theop-amp.

The embodiment of FIG. 2D provides an isolator circuit for ion detectorhaving a photodiode, comprising: a bias transistor having its emittedcoupled to a cathode of the photodiode and a collector coupled to aresistor; a pulse signal path coupled between the collector and thetransistor; an analog path coupled to an anode of the photodiode,wherein the analog path comprises an operational amplifier (op-amp)having one input coupled to the anode of the photodiode, a second inputcoupled to ground, and an output providing an analog signal responsecorresponding to the photodiode sensing photons. In the embodiment ofFIG. 2D the pulse signal path comprises a transformer having an inputside coupled between the resistor and the cathode of the photodiode, andan output side providing a digital pulse response corresponding tophotodiode sensing photons. In the analog signal path, the op-amp iscoupled to the anode of the photodiode in an inverting configuration,wherein the inverting input of the op-amp is coupled to the anode of thephotodiode. Also, a feedback resistor is coupled between the invertinginput and the output of the op-amp.

FIG. 2E illustrates an example of a replicator circuit for providingmultiple output signals, according to one embodiment. In thisembodiment, all of the output signals are coupled to the anode side ofthe photodiode. In the embodiment of FIG. 2E the cascaded transistorsdrive current through R1, R2, and R3; the current is Rf/Rx*(SiPMcurrent), because the transistors will all maintain the same base toemitter voltage drop. In this example, resistor R1 would be setrelatively low in value, so as to provide a large gain for the pulsedoutput. So, the embodiment of FIG. 2E provides a replicator circuit forion detector having a photodiode, comprising: a bias transistor havingits emitted coupled to a cathode of the photodiode and a collectorcoupled to a resistor; an anode of the photodiode is coupled to groundvia a reference resistor, optionally through a diode having its cathodeconnected to the reference resistor and its anode connected to the anodeof the photodiode; a base of a first second and third transistors isconnected in common to a tap positioned between the resistor and theanode of the photodiode; a second tap, positioned between the resistorand ground is connected in common to a first, second and thirdresistors; the first resistor coupled to an emitted of the firsttransistor, a second resistor coupled to an emitted of the secondtransistor, and the third resistor coupled to an emitted of the thirdtransistor; wherein a pulsed signal path is coupled to a collector ofthe first transistor; a first analog signal path is coupled to acollector of the second transistor, and a second analog signal path iscoupled to a collector of the third transistor.

As indicated, this feature is not limited to the use of the discloseddetector, but can also be implemented in systems using other detectors,such as a photomultiplier, a channel electron multiplier, etc. Anexample is illustrated in the solid-line callout. In the illustratedembodiment an electron multiplier 249 is used as a detector (theelectron multiplier 249 may be part of a photomultiplier or otherconventional sensor). An electron e-hits the first dynode, generating acascade of electrons from one dynode to the next, until they hit thecollector 253. The output of the collector 253 is coupled to the analogchannel. Conversely, the last dynode 256 is tapped and is coupled to thecounting channel. Consequently, again the two output signals arecorrelated to the same detection event, but are generated from differentelements of the system such that the two channels need not share thesame signal.

In the embodiment of FIG. 2A, an optional second analog output line isprovided, which can be used both for calibration and measurements. Theline leading the DC potential V1 to the electron emission plate 243 istapped and led to a current meter, e.g., an ammeter, or to an analogchannel, such as the one shown i9 n the dashed callout. This measurementprovides direct analog measurement corresponding to the arriving ions.There should be a correlation between the current measurements from theline leading to the electron emission plate and the output line of thedetector, such that the output of both can be used to study theconversion efficiency of the detector and thereby calibrate the digitalcount output of the detector.

FIG. 3 is a schematic illustration of an ion detecting scheme accordingto one embodiment of the invention. Detection section 330 shown in FIG.3 includes only the elements related directly to the detection and omitselement relating to extraction, charge compensation and filtering, whichshould be inserted upstream of and operated in conjunction withdetection section 330. Once the ion beam 320 passes the extraction,charge compensation and filtering elements, it enters the detectionsection 330 and passes through a beam shaping tube 331. The beam shapingtube 331 may comprise a plurality of lenses 332, which may be magneticlenses or electrostatic lenses, e.g., quadrupole lenses. In thisspecific embodiment, quadrupole lenses are used to shape and focus thebeam. A hexapole 333 is provided at the exit of the beam shaping tube331 to provide higher order optical correction to the beam. Optionally,a slit 333′ may be provided upstream of the hexapole 333 so as to filterout ions at the fringes of the beam. Alternatively, or additionally,another slit can be provided downstream of the hexapole 333.

The shaped ion beam then exits the beam shaping tube 331 and enters themass analyzer 335, which separates the ions according to theirmass-to-charge ratio. The mass analyzer 335 includes a spectrometer 336,which is followed by quadruples 332, a hexapole 333, and main magnet338. Generally, the extraction and beam shaping elements act on allcharged particles available to be extracted, but not all are of interestfor a particular analysis. Therefore, the spectrometer 336 is energizedto a selected voltage potential that enables a selection of a desiredmass-to-charge ratio band to pass through to the quadruples 332. Thequadruples 332 and hexapole 333, together with the main magnet 338,focus the beam onto a focal plane 360 (dash-dot line in FIG. 3). Thedetectors 340 are arranged spatially on the mass analyzer focal plane360. As ions exit the hexapole 333 their trajectory is changed accordingto each individual ion's mass to charge ratio, such that each ionarrives at a different point on the focal plane 360. The detectors 340are movably mounted on tracks 339, which parallels the focal plane 360.Mechanized control 350 may be included to control the positioning ofeach detector 340 individually. Thus, the multiple detectors 340 can bepositioned spatially along focal plane 360 to detect expected ionsaccording to each ion mass to charge ratio, within the band selected bythe spectrometer 336. Each of detectors 340 may be implemented accordingto any of the embodiments disclosed herein.

In a raster-scan secondary ion mass spectrometer, the primary ion beamis used to sputter material from the sample of interest. As the materialis sputtered, secondary ions are detected to thereby determine thematerial composition of the sample. As the primary ion beam scans thesample, it creates a crater in the sample, due to the sputtering ofmaterial of the sample. However, since the sputtering at the edge ofeach raster scan is not uniform, the data relating to the edge of theraster scan is discarded. FIG. 4 illustrates this condition, wherein theside view shows one line scan, of the many scan lines required to testthe entire sample. The scanning of the primary beam over the samplecreates a crater due to the sputtering of material from the sample. Therelatively flat bottom of the crater is the area where data is collectedto determine the material make-up of the sample. However, at the edgesof the crater, generally defined by the area where a slope is present,the sampling is inaccurate, so that the data is discarded.

FIG. 4 illustrates a condition wherein the ion sputtering causes acrater with different sputtering rate at the edges and at the center ofthe crater. In FIG. 4 a sample 400 is analyzed by scanning primary ionbeam 415 over area of interest 470. As the beam is raster scanned,illustrated by the arrows in the top view, a crater is created since thematerial makeup of the sample 400 is sputtered away. At the edges 472 ofthe crater the sputtering rate and trajectory of secondary ions isdifferent than from the flat bottom area 474 of the crater. Therefore,in the prior art data from the edges 472 has been discarded.

In one embodiment, one of the channels is used to perform measurementswhile the primary beam is scanning over the bottom area 474 of thecrater. For example, if the ion count is too high to use pulse countingmode, then the analog mode is used for measurement during the time theprimary beam scans the bottom area 474 of the crater. On the other hand,during the time that the primary beam scans at the edge 472 of thecrater, the data is not used for mass spectroscopy measurement. Instead,the simultaneous data from the pulse counting and analog mode channelsare used to calibrate the analog channel with respect the pulse countingchannel.

In another embodiment, one channel is used to perform the massspectroscopy measurement during the time that the primary beam scans thebottom of the crater. At the same time, the data obtained from the otherchannel is used to improve the data of that channel. Additionally, whenthe primary beam scans over the edge of the crater, the pulse-countingchannel is used to calibrate the analog channel. For example, a scalingfactor can be calculated from the difference between the output of thetwo channels.

In one embodiment, this calibration is performed at periods of time whenthe detected signal is safely within the pulse counting regime (e.g.,lower than about 10⁷ ions/sec) and for a sufficient period of timeduring an acquisition that a sufficient number of ion events areintegrated in the analog channel and counted in the pulse countingchannel to determine the average current per ion to within specificationand over a period of time over which the statistical processes involvedin detection are essentially stationary. According to this method, ascanning period is set and the beam is made to scan over a uniform areaof a sample. The analog channel and the counting mode channel areactivated to operate simultaneously. At the end of the scanning periodthe scanning of the beam is stopped and the outputs from the analogchannel and the counting mode channel are used to generate a calibrationfactor. This operation may be repeated a number of times, using the sameor different ion flow rates.

The method is useful when ion fluxes need to be measured (or may need tobe measured) which are too high for pulse counting be used. Thechallenge is to guarantee sufficient dual sampling on each detector onwhich analog mode is needed in the regime where pulse counting ispossible. According to some embodiments a suitable ion arrival rate isproduced at each detector for which analog mode is needed, during theraster scan and without interfering with acquisition.

According to one embodiment, electronic gating regions of the scan frameoffer a feasible means for implementing this approach. Measurement ofthe ion flux on each detector during the first N scans could be used todetermine a single attenuation factor which would cause the arrival rateat each of the detectors for which this calibration is needed to bewithin the safe pulse counting region. This attenuation factor would beachieved by some means in the secondary ion optics, such as deflectingor defocusing a beam on an aperture during the electronically gatedportions of a frame, and data collected on both the analog andpulse-counting channels. This data, with some moving average, would beused to track and calibrate the average current per pulse, which wouldthen be used to convert the analog signal during the measurementsegments of a frame into calibrated ion arrival rate.

It should be noted that the attenuation factor does not need to be knownaccurately enough to achieve a well-known ion arrival rate. It is onlyneeded that it is known well enough that an ion arrival rate can beachieved that is within a regime where pulse counting and analog outputare both possible. In any case, the goal is not to create a known ionarrival rate, but a measurable ion arrival rate which can be used alongwith the analog signal to calibrate the analog output. The gatingperiods may coincide with the times wherein the primary beam scans overthe edges of the crater.

In the above description, reference is made to the analog channel in thecallout of FIG. 2A. However, alternatively, the tap into the electronemission plate 243 can be used as the analog channel. Moreover, whenusing the analog channel of the callout in FIG. 2A, the tap of theelectron emission plate 243 can be used to further calibrate the analogchannel.

According to an embodiment, a system for performing secondary ion massspectroscopy is provided, comprising: a primary ion beam; opticsarrangement scanning the primary ion beam over a sample; a detectordetecting secondary ions emanating from the sample and generating adetection signal; a first detection signal line and a second detectionsignal line coupled to a first and second channels; wherein the firstchannel comprises elements configured to provide ion counting output andthe second channel comprises elements configured to provide integratedcurrent signal. The system may further comprise a controller programmedto receive the signals from the first and second channels and generate ameasurement data when the primary beam is at a central portion of itsscan and calibrate the first and second channels with respect to eachother when the beam is at an edge of its scan.

According to further embodiment, a method for operating a secondary ionmass spectrometer having a counting mode and an analog mode is provided,comprising: scanning a primary ion beam over a sample so as to sputtersecondary ions from the sample; collecting the secondary ions from thesample and simultaneously generating a first detection signal and asecond detection signal; applying the first detection signal to an ioncounting channel; applying the second detection signal to an ion analogchannel; and generating a calibration factor from the output of theanalog channel and the pulse counting channel. The first detectionsignal may be generated from a bias line coupled to the detector, whilethe second detection signal may be generated from the output of thedetector. When an electron-multiplier is used as the detector, the firstdetection signal may be generate from a line tapped to a dynode, whilethe second detection signal may be generated from the output of thecollector.

Further, a method for performing secondary ion mass spectrometry isprovided, comprising: sputtering secondary ions from a sample;collecting the secondary ions and forming a secondary ion beam; passingthe secondary ion beam through a mass analyzer so as to form multiplesecondary ion trajectories lined up on a focal plane; placing aplurality of detectors on the focal plane at expected secondary iontrajectories according to mass-to-charge ratio; at each of the detectorsperforming: generating a flow of electrons corresponding to receivedsecondary ions; generating a flow of photons corresponding to the flowof electrons; and generating a flow of electrical current correspondingto the flow of photons. The method may further comprise transporting theflow of photons inside a lightguide. Generating the flow of electricalcurrent corresponding to the flow of photons may comprise directing theflow of photons onto a solid-state photomultiplier. The method mayfurther comprise tapping the flow of electrical current and integratingover the tapping. The method may further comprise tapping the flow ofelectrons and measuring a current over the tapping.

It should be understood that processes and techniques described hereinare not inherently related to any particular apparatus and may beimplemented by any suitable combination of components. Further, varioustypes of general purpose devices may be used in accordance with theteachings described herein. The present invention has been described inrelation to particular examples, which are intended in all respects tobe illustrative rather than restrictive. Those skilled in the art willappreciate that many different combinations will be suitable forpracticing the present invention.

Moreover, other implementations of the invention will be apparent tothose skilled in the art from consideration of the specification andpractice of the invention disclosed herein. Various aspects and/orcomponents of the described embodiments may be used singly or in anycombination. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of theinvention being indicated by the following claims.

1. An ion detector comprising: an electron emission plate coupled to afirst electrical potential and configured to emit electrons uponincidence on ions; a scintillator coupled to a second electricalpotential, different from the first electrical potential, thescintillator having a front side facing the electron emission plate anda backside, the scintillator configured to emit photons from thebackside upon incidence of electrons on the front side; a tubularlightguide optically coupled to the backside of the scintillator andconfining flow of photons emitted from the backside of the scintillator;a photomultiplier coupled to the light guide and having an outputconfigured to output electrical signal corresponding to incidence ofphotons from the lightguide.
 2. The ion detector of claim 1, wherein thephotomultiplier comprises a solid-state photomultiplier.
 3. The iondetector of claim 2, wherein the solid-state photomultiplier comprises asilicon photomultiplier.
 4. The ion detector of claim 2, wherein thesolid-state photomultiplier comprises an array of avalanche photodiodeshaving outputs coupled together to provide a single electrical outputsignal.
 5. The ion detector of claim 1, wherein the tubular lightguidecomprises a reflective surface which directs the photons onto thephotomultiplier.
 6. The ion detector of claim 1, wherein thephotomultiplier is coupled to a rear surface of the tubular lightguide.7. The ion detector of claim 4, further comprising: a first signal linecoupled to the photomultiplier; a digital event counter configured toreceive an input from the first signal line and provide a discrete eventcount output; a second signal line coupled to the photomultiplierindependently of the first signal line; an analog integrator configuredto receive input from the second signal line and provide integratedanalog output simultaneously with the discrete event count output fromthe digital event counter.
 8. The ion detector of claim 7, wherein: thefirst signal line is coupled to an input bias line of thephotomultiplier; and, the second signal line is coupled to an outputline of the photomultiplier.
 9. The ion detector of claim 8, furthercomprising a transformer and wherein the first signal line is coupled toan input bias line through the transformer.
 10. The ion detector ofclaim 8, further comprising a capacitor and wherein the first signalline is coupled to an input bias line through the capacitor.
 11. The iondetector of claim 7, further comprising: a tap connected to the electronemission plate; and, a current measuring sensor connected to the tap.12. The ion detector of claim 1, further comprising: an electronmultiplier; a first signal line coupled to a dynode of the electronmultiplier; a digital event counter configured to receive an input fromthe first signal line and provide a discrete event count output; asecond signal line coupled to a collector of the electron multiplier; ananalog integrator configured to receive input from the second signalline and provide integrated analog output simultaneously with thediscrete event count output from the digital event counter.
 13. Asecondary ion mass spectrometer, comprising: an ion source providing aprimary ion beam; a secondary ion extractor collecting secondary ionssputtered from a sample by the primary ion beam; beam forming opticsreceiving the secondary ions from the secondary ion extractor andforming a secondary ion beam; a mass analyzer forming a trajectory ontoa focal plane; a plurality of ion detectors movably positioned along thefocal plane; wherein each of the ion detectors comprises: a firstsection configured to emit electrons upon impingement of the secondaryions; a second section configured to emit photons upon impingement ofthe electrons; a third section configured to emit an electrical signalupon impingement of the photons; and, a lightguide coupled on one end tothe second section and on another end to the third section and confiningtrajectory of the photons.
 14. The secondary ion mass spectrometer ofclaim 13, further comprising a spectrometer configured to allow onlysecondary ions within a desired band of mass-to-charge ratio to passthrough.
 15. The secondary ion mass spectrometer of claim 13, whereinthe third section of each detector comprises a plurality of avalanchephotodiodes having a common bias input line and a common output line.16. The secondary ion mass spectrometer of claim 15, wherein: the firstsection comprises an electron emission plate coupled to a firstelectrical potential and configured to emit electrons upon incidence onions; and, the second section comprises a scintillator coupled to asecond electrical potential, different from the first electricalpotential, the scintillator having a front side facing the electronemission plate and a backside, the scintillator configured to emitphotons from the backside upon incidence of electrons on the front side.17. The secondary ion mass spectrometer of claim 15, further comprising:a digital event counter coupled to the common bias input line andconfigured to provide a discrete event count output; and, an analogintegrator coupled to the common output line and configured to provideintegrated analog output simultaneously with the discrete event countoutput from the digital event counter.
 18. The secondary ion massspectrometer of claim 17, wherein the digital event counter is coupledto the common bias input line through a capacitor or a transformer. 19.The secondary ion mass spectrometer of claim 13, wherein the thirdsection comprises a solid-state photomultiplier coupled to the lightguide and having an output configured to output electrical signalcorresponding to incidence of photons from the lightguide.
 20. Thesecondary ion mass spectrometer of claim 13, wherein the third sectioncomprises an electron multiplier and further comprising: a digital eventcounter coupled to a dynode of the electron multiplier and configured toprovide a discrete event count output; and, an analog integrator coupledto a collector of the electron multiplier and configured to provideintegrated analog output simultaneously with the discrete event countoutput from the digital event counter.